Underground or above-ground cavities can constitute a problem as regards distribution of the stresses within a mass of soil or more generally within a body. Said cavities in fact constitute a discontinuity which, as such, does not cooperate in the distribution of the stresses within a volume.
Consider for example a cavity in a mass of soil located at the footing of a tall building. According to the laws of geotechnical engineering, in such a situation the load of the building is transferred to the soil unevenly, concentrating proximate to the surface that surrounds the cavity. In extreme cases, this load concentration can even reach and exceed the ultimate strength of the soil, with consequent collapse of the entire volume and accordingly of everything that rests thereon.
Various methods are known for filling underground cavities by using various types of materials.
In general, these methods have the goal of filling the entire volume of the cavity by means of setting liquid substances optionally mixed with inert solid filler material.
In particular, European Patent Application no. 0114448 discloses a method for partially or totally filling cavities by pumping a cement-based foaming material, which contains an inorganic expanded material such as pearlite and vermiculite. This method, despite being very expensive, does not ensure the complete filling of underground cavities with domed surfaces having an irregular geometry, since the expansion of the inorganic material is provided before pouring for filling and therefore the final distribution of the solidified mixture within the cavity follows a geometry which is governed only by the force of gravity.
Japanese Patents no. 09-228371 and no. 11-323904 disclose methods for filling cavities which are based on the separate use of granular solid material, optionally with the addition of lubricating foaming agents, for filling the easily accessible voids and subsequently of cement mortar or other materials in the fluid state, which are poured into the cavity in order to saturate the intergranular voids of the previously deposited solid material and fill the portions of the cavity that have not yet been reached. Even with these methods, saturation of the dome of the cavity is not possible, since the mortar or other material in the fluid state, due to the force of gravity, tends to settle on the bottom before solidifying. Moreover, the execution cost of this method can be very high, since complete saturation of the intergranular voids can entail the use of large quantities of mortar or other material in the fluid state. Finally, the considerable weight increase that the filling mixture produces on the ground underneath the filled cavity must not be neglected.
Another cavity filling technique is disclosed in Japanese Patent no. 2002-348849, according to which the filling mixture is injected into the cavity until preset injection pressures are recorded and in any case until said mixture exits from holes provided adjacent to the injection duct. This technique, in addition to suffering the disadvantages already noted with reference to the methods described above, can entail, in the case of underground cavities with fractured walls, very high execution costs, due to the disproportionate use of mixture with respect to the volume of the cavity to be saturated.
Another method which has the disadvantage of being unable to saturate the volume in the dome is the one disclosed in US Patent Application no. 2002/0015619. This method consists in plugging the underground cavities by using only solid inert material assisted by a lubricating foaming agent which facilitates its arrangement in the void.
Other known types of methods for filling underground cavities use expanding synthetic filler materials. For example, the methods disclosed in U.S. Pat. No. 3,478,520 and U.S. Pat. No. 4,744,700 use expanding synthetic material such as polystyrene, which increases in volume if it is placed in contact with heat sources. The methods for applying heat to the expanding synthetic material can be of various kinds. This method, which is unquestionably very expensive, is difficult to apply both as regards providing, if needed, a sufficient heat source and as regards distributing uniformly the heat within the cavity, allowing even expansion of the expanding synthetic material contained therein.